As is well-known, artificial ventilation (artificial respiration), also known as assisted or mechanical ventilation (assisted or mechanical respiration), replaces or supplements the activity of the inspiratory muscles of a subject, supplying the energy needed to ensure an adequate volume of gas for the lungs.
Usually a distinction is made between permanent assisted ventilation and temporary assisted ventilation.
Permanent assisted ventilation is usually performed by means of a negative ventilation system, using an air chamber which surrounds the chest, such as a “iron lung”, and which is rhythmically brought to a negative pressure so as to allow the aspiration of air into the airways and the lungs.
Temporary artificial ventilation is based instead on the use of positive pressure systems such as a ventilator or a simple oxygen-enriched air chamber, such as the Ambu bag or bag valve mask, which are connected to the airways and rhythmically operated, for example by means of a manual compression, so as to force air into these airways and therefore into the lungs.
In order to ensure an easy passage of the air inside the airways and isolation of the latter, the direct connection to the positive pressure source is performed by inserting a cannula into the larynx via the nose or the mouth, or by means of a tracheotomy. In other cases it is possible to perform simple actions on the airways or the laryngeal mask which is a substitute for the endotracheal tube.
Artificial ventilation is for example used in surgical operations which envisage curarization of the subject with consequent muscular paralysis and when the spontaneous respiration of the subject is no longer able to maintain the vital functions.
Among the many conditions which require artificial ventilation it is possible to mention also the presence of acute pulmonary lesion or apnea due to a respiratory arrest, including intoxication, but also the presence of a paralysis of the diaphragm as in acute crises tied to muscular dystrophy or amyotrophic lateral sclerosis or in the case of a spinal cord injury.
Assisted ventilation is also used in the case of reacutized chronic pulmonary diseases, acute respiratory acidosis or hypoxia, hypotension and shock, as in congestive heart failure or during sepsis.
All the methods of assisted ventilation share the general concept that part of the work is performed by the subject undergoing assisted ventilation, by means of generation of a negative muscle pressure (Pmusc) via the subject's inspiratory muscles, mainly the diaphragm, so as to recall air inside the lungs, and part of the work is instead performed by the assisted ventilation apparatus.
In particular, during the assisted ventilation, the subject is connected to a mechanical respirator or ventilator, via a tube positioned in the trachea, the flow of gas towards the subject (inhalation or inspiration) and from the subject (exhalation or expiration) being regulated by the ventilator itself.
In greater detail, the modern assisted-ventilation apparatuses envisage that, when the apparatus registers that the subject has started to breathe, the ventilator assists him/her, causing an increase in the pressure in the airways and consequently an increase in the flow of gas directed towards the subject (inspiration). In this way, the pressure needed to displace the gas is provided partly by the ventilator and partly by the subject, in a manner proportional to his/her muscle pressure (Pmusc).
In practice, it is therefore important to be able to estimate the spontaneous respiratory activity or the muscle pressure (Pmusc) being autonomously generated by the subject, since an underestimated assisted ventilation may cause tiredness and, more generally, problems for the subject undergoing the assisted ventilation, but also an excessive assisted ventilation may in any case cause, in addition to an increase in the oxygen consumption, problems for the subject. In particular, it is important to avoid the occurrence of ventilatory asynchrony between the subject and the assisted ventilation apparatus or ventilator, which has proved to be extremely damaging, in particular for subjects undergoing the assisted ventilation for a prolonged period of time.
At present, the reference standard for measurement of the muscle pressure (Pmusc) provided by a subject, and therefore an evaluation of the spontaneous breathing activity, is the measurement of the oesophageal pressure which requires in particular positioning of a balloon inside the oesophagus of the subject, connected to a transducer for detecting the oesophageal pressure, and on the basis of which the pressure inside the thorax and therefore the muscle pressure (Pmusc) provided by the subject is estimated.
This method, however, is rarely used in practice, not so much because it is invasive, but because it is technically complex with regard to interpretation of the data.
Other surrogate indices for evaluation of the spontaneous breathing activity, namely the respiratory work performed autonomously by a subject, have been developed; these include the so-called P0.1 (namely the pressure generated by the subject in the first 100 milliseconds from the start of inspiration during an expiratory occlusion) which is the most widely used index.
The most recent assisted-ventilation technologies, such as the NAVA® (“Neurally Adjusted Ventilatory Assist”) technology, instead have an approach to the assisted ventilation which is based on the neural respiratory emission, with detection of the electrical activity of the diaphragm.
It is in fact known that the respiratory action is controlled by the respiratory center of the brain, which establishes the characteristics of each breath, the duration and the amplitude. The respiratory center sends a signal along the phrenic nerve and activates the muscle cells of the diaphragm, causing contraction thereof and the descent of the diaphragmatic dome. Consequently, the pressure in the airways drops, causing a flow of air into the lungs.
The assisted ventilation apparatuses based on the NAVA® technology therefore receive a signal of the electrical activity produced by the diaphragm (Eadi) and use this signal to assist the respiration of the subject in synchronism, the work of the ventilator and that of the diaphragm being controlled by the same signal, such as to ensure simultaneous and synchronized cooperation between the diaphragm and ventilator. In this way the assisted ventilation is provided independently of conventional pneumatic sensors, such as the oesophageal balloon, and is not influenced by the air losses associated with the interfaces of the subjects, this being a very important condition for example in the case of a treatment of children who generally have a muscle pressure (Pmusc) which is too weak to be detected precisely by pressure or flow mechanisms.
Synchronization of the assisted ventilation with the respiratory activity associated with the spontaneous breathing activity of the subject in particular eliminates the lack of uniformity with the duration of the air inspiration and expiration by the subject, avoiding the risk of an inefficient effort and allowing lower assistance levels to be used.
Furthermore, the signal of the electrical activity as produced by the diaphragm (Eadi) is used to evaluate the conditions of the subject and in particular to decide when to perform the extubation of the subject, in particular when the amplitude of the electrical activity signal Eadi as produced by the diaphragm falls below a certain limit.